1. Field of the Invention
The present invention relates to a process for reconcentrating the overspray of one-component, aqueous coating compositions from spray booths having wet extraction by a multi-stage membrane process using the resulting permeant as booth water and using the resulting retentate as an aqueous coating composition or as a constituent of an aqueous coating composition.
2. Description of the Prior Art
The term "overspray" means those coating components that miss the target substrate during spray application of the coating and in the absence of particular precautions are lost.
Increasing environmental problems have prompted the recent development of a wide variety of processes aimed at reducing the volume of special waste arising from coatings overspray. Conventionally, the overspray diluted by the spray booth water is coagulated in collecting basins for disposal. The underlying concept in some of the newer processes for water-thinnable coatings dispenses with coagulation and, instead, reconcentrates the overspray with care for reuse as a coating composition.
DE-OS 2,353,469 describes reconcentrating the overspray by ultrafiltration. The diluted overspray flows past a semipermeable membrane such that the low molecular weight substances, in particular water but also low molecular weight dissolved binder components and auxiliary substances, pass through the membrane (i.e., the permeant or penetrant), while the principal components of the coating composition are retained by the membrane (i.e., the retentate).
DE-OS 3,428,300 describes the desirability of exclusively using demineralized water as the spray booth water in order to avoid overspray coagulation. The same objective is achieved by the process described in DE-OS 2,945,523 by the addition of emulsifying agents.
EP-A-0,141,171 discloses the possibility of continuous ultrafiltration. A portion of the mixture of booth water and overspray circulated in the spray booth circuit undergoes continuous separation and ultrafiltration. The permeate is returned to the booth water. The physical and chemical properties of the retentate is checked to determine its suitability for reuse as a coating composition. The reconcentrated overspray may be reused after, for example, dilution or concentration.
WO 91/09666 describes the reworking of heat-curable, aqueous alkyd or acrylic resins by ultrafiltration in the presence of an aliphatic amine to prevent the coating compositions from coalescing and a glycol derivative to adjust the ultrafiltration throughput. These additives, however, have the disadvantages of affecting the quality of the worked-up coating composition and also polluting the exhaust air from the booth.
The new literature (for example JOT 10 (1992) 32 to 38, JOT 3 (1992) 28 to 33) discloses that the prior art ultrafiltration leads to serious problems with many coating systems, thus making it impractical. These problems may be due, for example, to a marked foaming tendency due to low molecular weight emulsifying agents or possibly low molecular weight binder components which pass into the permeant and accumulate. The lost components may be crucial to the coating quality of the recycled material, making direct reuse of the retentate as coating composition impossible.
An object of the present invention is to develop a process which enables overspray from water-thinnable coating compositions to be reworked to form new coating compositions having substantially the original composition and, thus, the original quality.
It has now surprisingly been found that this object may be achieved with the specific multi-stage membrane process described in greater detail hereinafter.
Multi-stage membrane processes are known and are applied, inter alia, in the whey, sea water, oil emulsions or latex waste water reworking sectors (see, for example, M. Mulder "Basic principles of membrane technology" Kluwer Academic Publishers (1991), R. Rautenbach, R. Albrecht "Membran-trennverfahren: Ultrafiltration und Umkehrosmose" [Membrane Separation Processes: Ultrafiltration and Reverse Osmosis], Otto Salle Verlag, Frankfurt am Main (1981) and DE-OS 4,126,483). These references describe the many possibilities, including combining ultrafiltration with reverse osmosis. The permeant from ultrafiltration, which contains low molecular weight components, is reconcentrated by reverse osmosis. The retentate from reverse osmosis is fed again into the ultrafiltration feed stream.
EP-A-0,553,684 describes a multi-stage membrane process for reconcentrating the overspray from water-dilutable coating compositions in spray booths having wet flushing, in which the booth circulation water is preconcentrated in a continuous manner in an ultrafiltration unit and the permeant is returned as circulation water. Final concentration is performed batch-wise in a further ultrafiltration unit. The permeant from the final concentration stage may be reconcentrated by a reverse osmosis stage downstream. Because there is no provision for returning the retentate from reverse osmosis into the ultrafiltration feed, it is not possible with this arrangement to recover the overspray as a coating having essentially the original composition.
EP-A 0,567,915 describes the possibility of returning the retentate from the reverse osmosis stage either into the preconcentration stage or into the final concentration stage. Reconcentration of low molecular weight components by reverse osmosis is only practical up to relatively low concentrations of at the most 5%, due to osmotic pressure build-up, so that redilution occurs as a result of returning the retentate from reverse osmosis into the ultrafiltration stages. Because of the poor retention performance of ultrafiltration in the case of soluble low molecular weight coating components, the coating composition recovered in this process does not have virtually the original composition.